1. Field of the Invention
This invention relates to axle lift mechanisms for load-carrying vehicles and, more particularly, to lift mechanisms for automatically raising an axle/wheel assembly so as to disengage the wheels from a ground surface.
2. Description of the Related Art
Several types of vehicles, including semi-trailer truck vehicles and the like, have multiple sets of axle/wheel assemblies arranged in tandem so as to adequately support relatively heavy loads. To adjust the load support provided by these tandem assemblies, it is known to employ suspension systems utilizing adjustably pressurized air springs and the like.
When the vehicle is carrying a relatively light load, it is desirable to relieve the load transmitting relationship between the vehicle and one or more of the axle/wheel assemblies, and also to disengage the axle/wheel assembly from ground contact so as to reduce tire wear. To relieve load support when an air suspension system is employed, air pressure in the suspension can be reduced. To achieve disengagement of the tires from the ground surface, devices commonly referred to as axle lift mechanisms can be employed. Prior lift mechanisms utilized stressed mechanical springs located adjacent each wheel which acted directly between a vehicle frame and the axle. When the downward load forces exerted on the axle by the suspension system were relieved, such as through deflation of air springs, lifting forces exerted by the mechanical springs pulled the axle upwardly to a raised position.
In addition to lifting a set of wheels from engagement with a ground surface, an additional function of the retractable suspension is to permit the utilization of the vehicle on a railway in addition to use on a highway. FIG. 1 illustrates a typical configuration of a trailer 10 having a suspension 12 in a retracted position, and mounted on a bogie 14 adapted for railroad use. The retractable suspension 12 serves to both lift the trailer 10 a sufficient distance to permit the rail bogie 14 or other support to be placed beneath the trailer and thereafter retract the wheels from the ground to permit the trailer 10 to rest and ride upon the bogie 14 or other support.
An example of one such axle lift mechanism is depicted in FIG. 2 and generally corresponds to the mechanism disclosed in U.S. Pat. No. 3,771,812 to Pierce et al., issued Nov. 13, 1973 which is incorporated herein by reference. Referring to FIG. 2, a first prior art axle lift mechanism 16 is used with a vehicle having a wheel 18 mounted to an axle 20. The axle 20 is connected to a trailing arm 22 through a conventional U-bolt 24 and nuts 26. The trailing arm 22 is pivotably mounted through a pivot connection 28 to a bracket 30 rigidly secured to a vehicle frame 32. The opposing end of the trailing arm 22 is secured to a conventional air spring 34 mounted to the vehicle frame 32 through bracket 36.
When the air spring 34 is inflated, load from the vehicle is transmitted through the vehicle frame 32 to the trailing arm 22. The force exerted by the air pressure in air spring 34 pivots the trailing arm 22 through the pivot connection 28 so that wheel 18 engages the ground surface.
The first prior art axle lift mechanism 16 includes a lever arm 38 mounted to the vehicle frame 32 through a pivot connection 40. One end of a coil spring 42 is connected to the lever arm 38 above the pivot connection 40 with an opposing end of the spring 42 secured to the vehicle frame 32 by an adjustable connection 44. The axle lift mechanism 16 also includes a chain 46 having an upper end secured to lever arm 38 through a bolt 48 and a lower end secured to the axle 20 through a lug 50.
When the vehicle is carrying a relatively light load, it is desirable to disengage the wheel 18 from a ground surface. Accordingly, air pressure in the air spring 34 is decreased and the load forces normally exerted on the trailing arm 22 by vehicle frame 32 are correspondingly relieved. With the load forces on trailing arm 22 relieved, the tension of coil spring 42 rotates lever arm 38 in a counterclockwise direction. Correspondingly, link chain 46 lifts the wheel 18 and axle 20 to an appropriately raised position as shown in dotted line format in FIG. 2.
A second prior art axle lift mechanism 60 is shown in FIG. 3 and disclosed in U.S. Pat. No. 4,634,141 which is incorporated herein by reference. Lift mechanism 60 is used with a trailing arm 62 pivotably mounted at one end to a bracket 66 through pivot connection 64. Bracket 66 is rigidly secured to a vehicle frame 68. Although not shown in FIG. 3, the trailing arm 62 can also be connected to a vehicle axle and releasably coupled in a load supporting relationship to the vehicle frame 68 through an air suspension system in a manner similar to trailing arm 22 depicted in FIG. 2.
The lift mechanism 60 includes a lever arm 70 having its lower end coupled to the vehicle frame 68 through a pivot connection 72. A rigid bar 74 is pivotably coupled at one end to a top portion of the lever arm 70 in an over center arrangement through pivot connection 76. An opposing end of rigid bar 74 is received through an aperture of a spring cup 78 and secured thereto with a fastener 80.
The spring cup 78 and rigid bar 74 are mounted within a tubular housing 82 shown in sectional configuration in FIG. 3. A spring 84 is mounted within the housing 82 and has one end which bears against spring cup 78 and an opposite end which bears against a stationary lip 86 of housing 82.
A link 88 is also coupled to lever arm 70 and rigid bar 74 at the pivot connection 76 which is correspondingly connected to a clevis link 90 and lifting chain 92 comprising a series of links 94. The lifting chain 92 is connected at its lower end to the trailing arm 62 through an anchor 96.
The operation of the second prior art axle lift mechanism 60 is similar to the operation of the lift mechanism 16 previously described with respect to FIG. 2. That is, with the air spring (not shown) in a deflated state, the lift mechanism 60 will operate to lift the trailing arm 62 and interconnected axle and wheels. The lifting force is provided by forces exerted by the compression spring 84 bearing against the spring cup 78. The forces exerted on the spring cup 78 are translated through the rigid bar 74 to rotate the lever arm 70 through pivot connection 72. Rotation of lever arm 70 correspondingly exerts lifting forces on trailing arm 62 through bar link 88, clevis link 90 and lifting chain 92.
FIG. 4 shows a third embodiment of a prior art axle lift mechanism 100 also disclosed in U.S. Pat. No. 4,634,141 which is used on a vehicle having a normally lowered axle/wheel assembly to automatically lift and maintain the assembly in a raised position when the vehicle is not heavily loaded. A trailing arm 102 is connected to a vehicle axle (not shown) and an air spring (not shown). The trailing arm 102 is pivotably coupled at one end to a vehicle frame 104. The lift mechanism 100 includes a flexible strap 106 secured at one end to the trailing arm 102 and wound around a pivotable cam 108. The opposite end of the strap 106 is coupled to a rod 110 adjustably engaged with a spring cup 112. A tapered coil spring 114 which is coaxial with the rod 110 bears against the spring cup 112 and exerts forces on the rod 110 and strap 106 sufficient to rotate the cam 108 and lift the trailing arm 102 and axle (not shown) to a raised position when the pressure in the air spring (not shown) is relieved.
The flexible strap 106 acts as a cam linkage which operatively connects the coil spring 114 and the trailing arm 102. The flexible strap 106 is connected so that forces exerted by the coil spring 114 are translated coaxially through the flexible strap 106 to lift the trailing arm 102.
In prior art axle lift mechanisms disclosed in FIGS. 2-4, a spring mechanism is required to be located adjacent each wheel on each side of the axle in each suspension which requires the use of more than one spring and its associated components for each suspension. These additional components add materials cost and additional weight to the vehicle.